Extended range polarization target

ABSTRACT

A reflector, particularly useful for underwater use, comprising a base material having angled surfaces for retro-reflecting an incident beam of light back substantially parallel to the incident beam, and means for polarizing the reflected beam into the same circular polarization handedness as the incident beam.

I UllltEd States Patent 1 1 1 1 3,709,580

Fu itt et al. 1 Jan. 9 1973 54] EXTENDED RANGE POLARIZATION 3,401,5929/1968 Altman ..350 102 TARGET 2,362,573 11/1944 MacNeille ....350/1563,161,879 12/1964 Hannan et al.. ....350/152 1 Inventors: Ronald Fuglfl,San 3 Paul J- 1,240,398 9/1917 wood ..350/152 Heckman, Jn, an anta F3,563,633 2 1971 Mauer .350 157 both of Calif. FOREIGN PATENTS ORAPPLICATIONS [73] Asslgnee: The United States of America as representedby the Secretary f the 876,555 11/1942 France ..350/156 Navy 1,464,47511/1966 France ..350/156 [22] Filedi Ma 8, 19 Primary Examiner-DavidSchonberg Assistant Examiner-Paul R. Miller A 1.N 125665 [21] pp 0Attorney-Richard S. Sciascia, Ervin F. Johnston and John Stan [52] U.S.C1. ..350/157, 350/105, 350/152,

350/156 [57] ABSTRACT A reflector, particularly useful for underwateruse, 6 comprising a base material having angled surfaces forretro-reflecting an incident beam of light back substantially parallelto the incident beam, and means for [56] References Cmd polarizing thereflected beam into the same circular UNITED STATES PATENTS polarizationhandedness as the incident beam.

1,610,423 12/1926 Cawley ..350/152 2 Claims, 14 Drawing Figures TARGET CEXTENDED RANGE POLARIZATION TARGET STATEMENT OF GOVERNMENT INTEREST Theinvention described herein may be manufactured and used by or for theGovernment of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION This invention relates to an apparatus andmethod for an underwater viewing system wherein much backscattered andsidescattered light is present. When artificial lighting is usedunderwater, the visibility is often severely limited by light which isscattered from particulate matter and microorganisms in the water backtoward the observer or photo-optical receiver. Attempts to reduce theeffects of this backscattered light have resulted in investigations intosuch advanced viewing systems as volume scanning, range gating, andpolarization discrimination. Of these advanced systems, polarizationdiscrimination is the easiest to implement.

A polarization discrimination system uses a right circularly polarized(RCP) source of illumination and a right circular analyzer. Thebackscattered light will in general be left circularly polarized (LCP),and as such will be blocked by the right circular analyzer. The lightwhich returns from most underwater targets will be unpolarized, however,and a portion will be transmitted by the analyzer.

One problem encountered with the polarization discrimination system isthe absorption of unpolarized target light. Since it is necessary toeliminate the polarized backscatter, at least half of the unpolarizedtarget light will also be absorbed. With available analyzers theabsorption is approximately 63 to 77 percent of the incident value. Theoutput power of the light source could be increased to compensate forthis loss, but the additional power drain would be a disadvantage foruse on underwater vehicles which have limited energy supplies.

SUMMARY OF THE INVENTION This invention relates to an apparatus andmethod for viewing a target area, generally underwater, which permitsviewing or otherwise optically detecting the target at ranges whichexceed those possible by using presently available targets. Theapparatus includes a source of circularly-polarized light forilluminating the target area; a target, at least some portion of whichhas a retro-reflective surface; a birefringent quarter-wave platecovering the retro-reflective surface of the target; and a circularpolarization plate positioned between the viewing lens and the lightreflected from the target. The arrangement permits the transmissionthrough the polarization sheet, or polarization plate, of lightreflected from the retro-reflective surface which has the samehandedness as the light produced by the source.

Since the target will return RCP light rather than unpolarized light,when illuminated with RCP light, approximately twice as much targetlight will be transmitted by the polarization analyzer. The polarizationtarget will thus appear brighter than conventional targets and can bedetected at greater ranges. Since this type of target also redirects ahigher percentage of incident light back to the observer, orphoto-optical receiver, it will be brighter than conventional targetseven when polarization discrimination is not used. Target materials ofthis type could be bonded to a variety of underwater objects. Sincethese objects will be visible at greater viewing ranges, they can belocated easier than objects not bonded with such materials.

STATEMENT OF THE OBJECTS OF INVENTION An object of the invention is toprovide a reflector material and method for viewing underwater objectswhich permits more efficient use of the source of light than prior artmethods.

Another object of the invention is to provide a reflector material andmethod suitable for use in an environment where much scattering ofreflected light is present.

Still another object is to provide a reflector material which, whenbonded or attached to an underwater target, permits locating the targetat much greater distances than targets not using this kind of reflectorsurface.

Other objects, advantages and novel features of the invention willbecome apparent from the following detailed description of theinvention, when considered in conjunction with the accompanyingdrawings, wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic view of aprior art circular polarization technique for the elimination ofbackscatter.

FIG. 2 (A-E) is a set of cross-sectional diagrammatic views showing thepolarization characteristics of un derwater targets for incidentcircularly polarized light.

FIG. 3 is a diagrammatic view showing an experimental arrangement forthe measurement of parameters of target radiance.

FIG. 4 is a pair of graphs showing the transmission factor of circularpolarization analyzers for two different Polaroid materials.

FIG. 5 (A-D) is a set of diagrammatic views showing qualitatively therelative visibility of target returns with and without polarizationdiscrimination.

DESCRIPTION OF THE PREFERRED EMBODIMENTS One of the problems encounteredin underwater viewing is the loss of contrast due to the presence ofbackscattered light. It has been shown that circular polarizationdiscrimination techniques are effective in some cases as a means ofimproving target visibility or extending the viewing range.

FIG. 1 illustrates the main components of an extended range or contrastimprovement system 10 of the prior art which utilizes circularlypolarized light. A source of unpolarized light 12 propagates a beam ofunpolarized light 14 through a right circular polarizer 16, whereupon itbecomes right circularly polarized (RCP) light 18. The RCP light 18,after impinging upon the target 22, becomes depolarized target light 24.There is also a considerable amount of left circularly polarized (LCP)backscattered light 26 produced.

In many cases of underwater viewing, much of the backscattered light 26is due to single scattering by suspended particles and micro-organismsin the water. If right circularly polarized RCP light 18 is used forillumination, much of the backscattered light 26 is left circularlypolarized LCP, and can thus be blocked with a polarization analyzer,right circular analyzer 28. The light 24 returning from most underwatertargets 22 is unpolarized, however, and a portion of this target lightis transmitted by the analyzer 28, to become polarized light 32 when itenters the detector 34.

The polarization discrimination system shown in FIG. 1 has twofundamental disadvantages. The first is absorption of initiallyunpolarized source light 14 by the polarizer 16 (approximately 63percent for Polaroid HNCP37), and the second is absorption of returningtarget light 24 by the analyzer 28 (another 63 percent).

By using an improved type of polarizer 16, much of the power loss causedby the initial polarization can be eliminated. The second problem,absorption at the analyzer 28, is however, an inherent feature of thepolarization discrimination system 10. Since it is necessary toeliminate polarized backscatter 26, at least half of the unpolarizedtarget light 24 will be absorbed.

If targets are designed to return RCP light rather than unpolarizedlight, approximately twice as much of this target light will betransmitted by a right circular polarization analyzer. If these targetsalso re-direct a higher percentage of incident light back toward thereceiver, they will offer improved contrast and increased viewing rangeseven without the use of polarization techniques.

In underwater viewing situations, targets can be classified asuncooperative and cooperative. The former would represent the situationwherein an object is to be found which has not been treated for ease ofoptical detection e.g., enemy hardware, or the treatment to which theobject has been subjected has deteriorated. A cooperative target wouldbe one that is specially treated to effect easy optical detection orrecognition.

Referring now to FIG. 2, FIG. 2A shows what happens when RCP light 18strikes an uncooperative, diffuse, target 22. The reflected light 24becomes depolarized.

Two different approaches have been taken to design targets which returna large amount of RCP light to the receiver. The first involves the useof a two-reflection retroreflector 42 which redirects incident light 18back toward the receiver (not shown) for a variety of targetorientations, as shown in FIG. 2B. A retro-reflector may be defined as areflector wherein the directions of the incident light and the reflectedlight are approximately parallel. The polarization nature of this target42 can be explained by resolving the incident RCP light beam 18 into twolinearly polarized component beams which have orthogonal electricvectors oscillating at 90 out of phase with each other. For a singlereflection from a metal 42 or metallic surface at near-normal incidence,a phase shift of 180 will occur between these two component beams. Sincethey are originally out of phase by 90, this additional phase shiftproduces light which is LCP 44. A second reflection at near-normalincidence introduces another 180 phase shift and the light becomes RCP46. Although the actual light beam 18 does not strike the metallicsurface of the retroreflector 42 at normal incidence, it can be shownthat the approximation to 180 phase shifts is reasonably valid exceptfor angles of large incidence. To provide retroreflectioncharacteristics in more than one plane, the actual target 42 is an arrayof four-comer reflectors. The angles of the target 42 in a planeperpendicular to that shown in FIG. 23, would generally be at also, asshown in FIG. 2E.

The second approach makes use of birefringent quarter-wave materials andsingle reflections. One embodiment 50 includes a target which consistsof a quarter-wave plate 52 and a dimpled metallic reflector 54, as shownin FIG. 2C. Typical dimpling would consist of indentations one-sixteenthinch deep with a radius of curvature of one-eighth inch, and spacedonefourth inch apart in a regular hexagonal pattern. The polarizationnature of this target 50 can be explained by again resolving theincident RCP light 18 into two linearly polarized components which are90 out of phase. The two-way path through the quarter-wave plate 52produces two 90 phase shifts between these components, and one phaseshift occurs when the light 18 is reflected at near-normal incidence.Since the total phase shift produced by the target 50 is 360 (or 0), thereturning light 46 will be RClP.

With respect to the type of light source to be used with the reflectormaterial of this invention, it need not be monochromatic, although thequarter-wave plate 52 is more effective with this type of light. Forunderwater use, a green light source is generally used, and a laserlight source would be particularly advantageous. The dimpled surface ofthe reflector 54 has a semi-diffusing action on the incident light 18which allows the target 50 to be visible from more than one viewingposition. It should be pointed out that the angle 6, between theincident beam 18 and the reflected beam 46,is exaggerated in thisfigure.

A target 60 which utilizes a quarter-wave plate 52 and a Scotchlitereflective sheeting, retro-reflective, material 62 is shown in FIG. 2D.Scotchlite reflective sheeting is a trademark of, and manufactured by,the Minnesota Mining and Mfg. Co., Reflective Products Division, St.Paul, Minn. 55119. The Scotchlite sheeting 62 is composed of microscopicglass spheres, or beads, 64 of high refractive index which are partiallyimbedded in a tough plastic reflective substrate 66. The incident light18 passes through the quarter-wave plate 52, is focused onto thereflecting substrate 66 by the spheres 64, and returns back through thequarter-wave plate in a direction approximately parallel to the incidentbeam. Although the polarization characteristics are the same as for thepreviously described model, embodiment S0 of FIG. 2C, this target 60 ismuch more efficient due to its more complete retro-reflective action.

Referring now to FIG. 3, the measured target radiance is given by i ()p(t 2,) (1) where H, is the irradiance incident on the target, p is thereflection distribution function, a is the volume attenuationcoefficient of the medium, and r is the distance from the target to thereceiver. If the viewing system employs circular polarizationdiscrimination, the transmission factor of the analyzer is given by T(),

where is the degree of circular polarization for the returning targetlight. If polarization techniques are not used, T() is equal to unity.

The degree of circular polarization may be defined operationally asfollows: A beam containing a mixture of unpolarized and circularlypolarized (CP) light is passed through a birefringent quarter-wave plateand a linear analyzer. The quarter-wave plate converts any CP light tolinearly polarized light, but does not affect the unpolarized light. Bymeasuring the degree of linear polarization for this beam we can thusobtain a measure of circular polarization for the original beam. Withthe preceding arrangement, the degree of circular polarization is givenby nuz.r min)/( ma.r mIn)]a where H and H are maximum and minimum valuesfor the irradiance which is transmitted by the linear analyzer as it isrotated. This function ranges from zero for unpolarized light to :1 forcompletely circularly polarized light. To differentiate between rightand left circularly polarized light, RCP values are considered to bepositive and LCP values negative.

By measuring in this manner, it is not necessary to assume a particularfunctional relationship between 5 and T(), the transmission of acircular analyzer. Once such transmission curves are known, however, thedegree of circular polarization is easily determined by measuring T().

Referring now to FIG. 3, equation 1 indicates that the measured targetradiance can be increased by increasing p(9 for values of 6,, 0 and qb,which are commonly used with underwater viewing systems. If circularpolarization techniques are being used, then T() should also be as highas possible.

The transmission characteristics of Polaroid I-INCP37 and I-IGCP21analyzers were measured as a function of degree of circular polarizationfor the incident light. These curves, shown in FIG. 4, indicate thatT(), and thus the measured target radiance, is a linear function of thedegree of circular polarization.

The empirical equations which represent these curves are:

T()=0.36+0.37 (HNCP37), (3)

and

T()=0.23 {+0.23 From Eqs. 3 and 4, we have )l It may be shown that asdetermined by this method, is identical to the fourth normalized Stokesparameter.

Qualitative and quantitative investigations were performed to comparethe efficiencies of several underwater targets.

Qualitative representations of photographs of various targets are shownin FIG. 5. The source-receiver separation angle 6, was fixed atapproximately (see FIG. 3), and identical exposure times and aperturesettings were used for all three photographs. Circular polarizationdiscrimination was effected by placing a Polaroid HGCP2l circularanalyzer in front of the camera lens. From this figure severalqualitative observations can be made. When circular polarizationdiscrimination is used:

(HGCPZI 4 l. RCP light which undergoes a single reflection iseffectively blocked by the analyzer. A single reflection at near-normalincidence produces a phase shift between orthogonally polarizedcomponents, and thus converts RCP light to LCP light. See FIG. 2B.

2. The brightness of a typical white diffuse target,

target 2, is considerably reduced.

3. Ordinary Scotchlite, target 3, is no brighter than the white diffusetarget, target 2.

The brightness of a dimpled reflector, target 1, is

considerably improved by covering it with a quarter-wave birefringentplate, target 4. For 0 0, however, this combination is not as bright asthe white diffuse target, target 2. Refer to FIG. 3 for a definition of6 the polar orientation angle.

. Return from the two-reflection retro-reflector, target 5, is excellentfor 0 0, but rapidly decreases as 6, increases.

. The Scotchlite and quarter-wave plate combination, target 6, isconsiderably brighter than the white diffuse target, target 2. Thisbrightness does not significantly decrease as 0 increases.

To determine the contrast improvement or range increase obtained byusing improved targets, it was necessary to measure quantitatively thetarget radiances as a function of the source-receiver separation angle0,, and the polar orientation angle 0 See FIG. 3. Specification of anazimuthal orientation angle da is only necessary for the two reflectionretro-reflector shown in FIG. 2E, since the other targets possessazimuthal symmetry. For this target, (b is arbitrarily set equal to zerowhen two sides of the rectangular retroreflector array (FIG. 5) areparallel (or perpendicular) to the plane of incidence.

Measurements were taken for four targets which, for simplicity, aredesignated by the letters A, B, C, and D as follows, in accordance withthe designations shown in FIG. 2:

A white diffuse target,

B two reflection retro-reflector,

C dimpled reflector and quarter-wave plate,

D Scotchlite sheeting and quarter-wave plate.

The range of each of these targets was fixed at approximately 4.2meters, and relative radiance was measured with a Gamma Scientific model2,000 telephotometer. The illuminating source was circularly polarizedby use of I-IGCP2l Polaroid, and polarization discrimination waseffected by placing an HNCP37 Polaroid analyzer over the telephotometerlens.

The relative radiance of these targets was measured both with andwithout polarization discrimination for values of 62 between 0 and 50,and values of 6, between 174 and 10. For a linear source-receiverseparation distance of 20 inches, this range of values for 9 correspondsto a range of viewing distances between 16.7 meters (6 l.74) and 2.9meters (6 10).

It was determined that targets D, Scotchlite and quarter-wave plate; B,two reflection retro-reflector; and C, dimpled reflector andquarter-wave plate; were all brighter than target A, white diffusetarget, for small polar orientation angles. It was also determined thatfor a given target orientation, that is, for fixed values of 8 0 and 0,the improvement is generally greater with polarization discriminationthan it is without it.

For many orientation and separation angles, how ever, two of thesetargets, B and C, are less bright than target A. When polarizationdiscrimination is not used, target C is brighter than target A only forvalues of which range from zero to approximately 10. When polarizationdiscrimination is used, this range increases to approximately 15. When 00, target B is brighter than target A for values of 0 up toapproximately 25. When polarization discrimination is not being used,there is also a large increase in radiance for 9 45.

From FIG. 28 it can be seen that when 0 45, the bisector of 0 will benormal to one set of reflecting facets on the target, and light willreturn to the receiver after being singly reflected. Since RCP lightbecomes LCP when it is singly reflected at near-normal incidence, muchlower radiance values are obtained for 6 5 45 when polarizationdiscrimination is employed. lf 4) is not equal to zero (or multiples of90), the measured radiance of target B is greatly reduced sincetwice-reflected light does not return to the receiver. It can also beshown that with d) 45, the measured radiance of this target very rapidlydrops below that of target A as 0 is increased.

The most significant improvement was obtained with target D, Scotchliteand quarter-wave plate. This target was brighter than any of the othersfor a wide variety of target orientations. Due to its retro-reflectingcharacteristics, the improvement is most pronounced for smallsource-receiver separation angles (long ranges), and decreases as thisangle increases.

Measurement of the effect of polarization discrimination on the measuredradiance of targets A and D show that approximately 71 percent of thelight returning from target D is transmitted by an HNCP37 analyzer. Fortarget A, approximately 37 percent of the returning light passes throughthe analyzer.

By referring to the T() versus g curve for l-lNCP37 Polaroid shown inFIG. 4, it can be seen that the light returning from target D is almostcompletely RCP, and that returning from target A is unpolarized. Thesepolarization characteristics are largely independent of thesource-receiver separation and the polar orientation angle.

In summary, quantitative radiance measurements have been made for threenewly developed targets, targets B, C, and D of FIG. 2, and for onewhite diffuse comparison target, target A. These measurements indicatethat significant range increases are possible when one of these targets,target D, Scotchlite and quarter-wave plate, is used with either aconventional viewing system or with one which employs polarizationtechniques to discriminate against backscatter.

For source-receiver separation angles between l.74 and 10, the radianceof this target D was between 1.79 and 33.2 times that of the comparisontarget, target A. The calculated range increases afforded by using thistarget are between 0.5 and 1.5 attenuation lengths. The specificradiance improvement factor, and associated range increase, for aparticular viewing system depends on whether or not polarizationdiscrimination is employed and on the source-receiver separation anglesinvolved. In general, greater radiance improvements are obtained whenpolarization techniques are employed, and when source-receiverseparation angles are small.

If a target is constructed from Scotchlite and very thin, flexiblequarter-wave material, it could be bonded to a variety of underwater obects, such as, test torpedoes, diver marking aids, tools, divers suits,underwater habitats, etc. These objects would then be visible at greaterviewing ranges, and could be more easily located.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

l. a reflector, particularly useful for underwater use, comprising:

a base material;

the base material having a single-reflection retroreflective, dimpled,surface, the dimples being in the range of 1.5mm deep, with a radius ofcurvature of approximately 3.0mm, and spaced approximately 6mm apart;and

a sheet of birefringent quarter-wave plate disposed upon the dimpledsurface.

2. A reflector particularly useful for underwater use, comprising:

a plastic substrate;

a plurality of transparent glass, microscopic, spheres imbedded in thesubstrate;

the surfaces of intersection of the spheres and the substrate forming areflective surface; and

a sheet of birefringent quarter-wave plate disposed upon, or adjacentto, the imbedded spheres, parallel to the substrate.

1. A REFLECTOR, PARTICULARLY USEFUL FOR UNDERWATER USE, COMPRISING: ABASE MATERIAL; THE BASE MATERIAL HAVING A SINGLE-REFLECTIONRETRO-REFLECTIVE, DIMPLED, SURFACE, THE DIMPLES BEING IN THE RANGE OF1.5MM DEEP, WITH A RADIUS OF CURVATURE OF APPROXIMATELY 3.0MM, ANDSPACED APPROXIMATELY 6MM APART; AND A SHEET OF BIREFRINGENT QUARTER-WAVEPLATE DISPOSED UPON THE DIMPLED SURFACE.
 2. A reflector particularlyuseful for underwater use, comprising: a plastic substrate; a pluralityof transparent glass, microscopic, spheres imbedded in the substrate;the surfaces of intersection of the spheres and the substrate forming areflective surface; and a sheet of birefringent quarter-wave platedisposed upon, or adjacent to, the imbedded spheres, parallel to thesubstrate.